Much attention has been paid recently to near eye displays (NED), specifically virtual reality (VR) and augmented reality (AR) viewers, particularly in the area of consumer electronics. A NED produces virtual image or scene, when positioned in front of and near the eye. In some NEDs, the virtual scenes are binocularly positioned in front of the left and right eyes so that the virtual scenes are perceived by human stereoscopic vision as three-dimensional scenes. In AR NEDs, the user of the NED can see through the optics that are part of the virtual scene delivery system producing the illumination in the design eye box and see the virtual scene content superimposed on the real world. VR NEDs produce virtual scenes that cannot be seen through.
A virtual scene is one that is presented to the eye of the observer with a lens or other optical conveyance so that it is collimated or nearly collimated. These types of devices are known as Near Eye Displays (NEDs) and Head Up Displays (HUDs). NEDs and HUDs are similar in that they both present virtual scenes to the eyes of display users. NEDs produce a virtual scene of both digital and analog content with pixel and raster scan display technologies.
In HUDs, the virtual scenes are presented to the display by reflecting off of an optical component or surface referred to as a beam combiner, positioned at a distance typically more than 30 cm from the eyes and not attached to the display head. The HUD beam combiner can be thought of a window that when the display user looks through it, the virtual scene of the presented information is superimposed on the real world seen through and on the opposite side of the beam combiner similar to the AR NEDs.
The virtual scene from NEDs and HUDs can be perceived by the eye if the entrance pupil of the eye is positioned within the area of the NED where the illumination from the virtual scene is projected by the optical design of the NED. This area is often referred to as the design eye box of the NED. The uniformity of the illumination in the plane of the eye entrance pupil can vary in intensity depending on the position within the plane bounded on the outside by the edges of the design eye box. In some embodiments, a small (for example, 2 to 5 mm) diameter aperture is selected to simulate the entrance aperture pupil of the eye.
As NEDs transition from novelties to mainstream products, achieving dependable and predictable performance will become increasingly critical. For consumer NED systems, this is necessary in order to ensure a consistent user experience, and to guarantee that products reliably meet both cosmetic and performance standards that are congruent with the manufacturer's brand identity. For commercial NEDs, particularly those for avionics and military uses, achieving a specified performance level is often critical to the correct functioning of the device in its application.
However, a lag in NED testing capabilities occurs because the optical systems employed in these devices, and the way they are utilized by the viewer, are both somewhat unique. As a result, traditional optical metrology equipment cannot be simply adapted to the demands of NED testing and thus a new approach is necessary.
All NEDs include three essential elements. The first is a display or source of some type, which generates light or a scene. The second is an optical system which projects the light into the viewer's eye(s). These optics are necessary because most people cannot comfortably focus on an object which appears to be close (e.g. less than two inches) to the eye. Thus, the optics create a virtual scene of the display source which appears to be at a sufficient distance for easy accommodation, and also allows for stereoscopic scene fusion if the device provides a 3D scene. Furthermore, the optics may combine the display output with a view of the actual scene surrounding the user (AR), or entirely block off any view of the true environment (VR). The final component of a NED is the mechanics to hold the first two elements on, or in front of, the viewer's head and position them properly with respect to the user's eyes.
There are already quite a number of different design forms for NEDs in use or in development. These vary substantially in terms of the technology used for scene generation and the configuration of their optics. Nevertheless, whatever the underlying design for a particular NED, the combined output of the display and optics can be characterized by a few key parameters.
Exit pupil is the area of the region of the volume or area of light formed by the NED optics. If the eye is placed anywhere within the exit pupil, it will see the entire field of view of the display scene or image. In some embodiments, the exit pupil is in the range of 15 to 20 mm wide, since this size provides for some tolerance in the placement of the eye relative to the optics, and also allows for the variations in inter-pupillary distance which naturally occur in the population.
Eye box, (e.g., ISO9241-302 &-305 defined as Qualified Viewing Space) is a volume that contains the NED exit pupil and extends back towards the eye as well as forward toward the NED device. If the eye is placed anywhere within the eye box, the viewer will see entire the field of view of the display. Eye relief refers to the distance from the exit pupil to the nearest surface of the NED optics. In some embodiments, eye relief is designed to be large enough (>20 mm) to allow space for the eyes of users who wear eyeglasses to access this point. Field of view (FOV) is the horizontal and vertical angle, which the display appears to subtend as seen by the viewer's eye.
The optical parameters most typically measured for NEDs and for most types of displays include output spatial uniformity, contrast ratio and absolute luminance and color accuracy. For larger displays, such as flat panel displays and projectors, uniformity is traditionally measured using an imaging colorimeter or some other type of calibrated, camera based apparatus. Absolute luminance and color is usually measured using a spectroradiometer with narrow field of view collecting optics (e.g. a telescope).
For optical radiance measurements in a radiometer or spectroradiometer, focusing optics (objective lens) and a measurement area defining aperture (field stop) assures that a specific area on a surface, for example, an area of pixels in a device under test such as a liquid crystal display, is isolated for measurement. The defined measurement area, seen in a view finder by a view finder image sensor is the same as that to which the radiometer image sensor is responding. For precise spectroradiometric measurements of the virtual scene in NEDs and HUDs, the objective lens entrance pupil diameter needs to be smaller than the projected illumination area that the display users eye is to be positioned for optimum performance of the display (design eye position) and in some cases smaller than the typical human eye iris. The center of the entrance pupil also needs to be located at the display design eye position. Selecting different areas of the virtual display scene field of view requires the angular pointing of the spectroradiometer with only pivot motion of the entrance pupil relative to the design eye position.
However, it is not practical in the radiometer art to provide some manner in which a user can see from the viewpoint of the radiometer image sensor, without utilizing a view finder. Means are needed for assuring that what the view finder sees is substantially identical to what the radiometer image sensor is responding to. If the images seen by the view finder and the radiometer image sensor do not correspond, the radiometer output may be inaccurate and meaningless. An apparatus for performing such a function is described in the U.S. Pat. No. 3,813,172, the entire contents of which is hereby expressly incorporated by reference. In that apparatus, an objective lens is focused on a surface and focused on to a beam splitting aperture wheel. A portion of the focused light is directed to an opto-mechanical view finder, and the light directed through the aperture is delivered to a radiometer photo sensor. This radiometer illustrates principles utilized in maintaining the view seen by the view finder in registration with the view seen by the photo sensor.
An individual pixel in a color display may be as small as 1 arcminute by 1 arcminute In order to take a measurement from the pixel, the portion of a virtual scene, i.e., the virtual scene plane on which the radiometer must focus, must be less than 1 arcminute by 1 arcminute. Further, it is unsuitable to use a view finder eyepiece that has inadequate resolution to resolve the details of a single pixel or sub-pixel for the purpose of determining the depth of contrast produce by the display. Also, some forms of emitters or reflector have some polarizing effect on the light therefrom. Certain optical arrangements can exacerbate any effects due to this phenomenon. Augmented Reality and Virtual Reality displays present additional challenges in the use of spectroradiometers for luminance and color measurement mainly resulting from the requirement that the optical collection system have an entrance pupil smaller than the design eye box which leads to low light levels available for the spectral radiance measurement.
Moreover, the collection optics preserve the spatial information of the source. This is necessary in order to make accurate color and luminance (the intensity of light emitted from a surface per unit area in a given direction) measurements of any given sub-region of the display. For example, it might be desirable to measure the characteristics color and luminance of a single displayed character or symbol. Therefore, integrating spheres, fiber optics, or any other collection optics that do not preserve angular information are not useful for this type of NED measurement.
Another difficulty with employing traditional spectroradiometer collection optics with most NEDs is that they are typically too large to fit within the available space. Specifically, many NEDs are built into eyeglasses, goggles, headsets or helmets, enabling them to be worn by the user. This means that the collection optics for any test gear must be able to fit into the same space as the user's head or eyes. Indeed, in many cases, the test system should even be small enough to allow it to independently access the output of the left and right eye positions of the NED display. Thus, the ideal optics for NED testing should have a form factor which is about half the size of the available space for the viewer's eyes.